The invention relates to the filtration of soot/ash water slurries. Such slurries are obtained in a number of chemical processes such as chemical gasification processes using heavy hydrocarbon feedstocks. More specifically the invention concerns an improved partial oxidation, gasification process where the soot/ash water slurry emerging after the carbon removal step of such process is filtered off and disposed of directly.
Partial oxidation processes for hydrocarbon feedstocks were developed and commercialized during the 1950's. The best known processes, such as the Shell gasification process and the Texaco gasification process, have been utilized in a number of commercial plants.
Such gasification processes utilizing hydrocarbon feedstock normally comprise three principal steps:
Gasification, in which the feedstock is converted into raw synthesis gas in the presence of oxygen and steam. PA1 Waste heat recovery in which high pressure steam is generated from the hot gases leaving the reactor, and PA1 Carbon removal, in which residual carbon contained in the reactor outlet gas is removed in a multistep water wash. PA1 1. The carbon is recovered by means of pelletising, where a distillate or a residual fuel oil is used to form agglomerates with the carbon particles. The pellets can easily be separated from the water and are recycled to the reactor and/or burned in a carbon oil furnace. PA1 2. Alternatively the carbon slurry is contacted with naphta in an extractor to form naphta soot agglomerates. The agglomerates are subsequently decanted or sieved-off and converted into a pumpable mixture together with the feedstock and recycled to the reactor.
Hereby the unburned carbon from the gasifier will be made into a carbon slurry, an aqueous suspension containing soot and a significant amount of ash, depending on the feedstock, which has to be further processed and recycled.
Thus a serious drawback of such processes is that a certain percentage of the feedstock is not gasified and remains in the form of soot mixed with appreciable amounts of ash from the heavy hydrocarbon feedstock.
Traditionally the soot recovery is handled by two alternative routes:
However, as the soot is heavily contaminated with ash the disposal of the soot/ash mixtures has gradually become the most serious problem for such gasification processes.
The possibility to separate the soot/ash mixture by filtration and to dispose of it directly is considered as an attractive solution, but has not been applied on a large scale.
More specifically, filtration has been used to recover soot for special applications, such as absorbent carbon, conductive carbon and carbon black. However, such applications will not solve the disposal problems in a large gasification plant.
The soot/ash slurry from the carbon separation step will normally contain 0.5-3% unburned carbon and 0.1-2% ash. The ash contains appreciable amounts of Ni, Fe and V. The filtering of such a slurry is extremely difficult. As the water is removed the slurry is gradually turned into a soapy paste which is very difficult to handle by normal filtration means. The final water content of the filtercake will be 85% or higher and the pasty consistency of such a filtercake makes it unsuitable for further handling.
Subsequent combustion will result in excessive caking and the high combustion temperatures needed to burn the soot create severe environment and corrosion problems.
To overcome these handling problems it has been proposed to add other solid matter to the slurry. Thus according to DE-A-4003242 a soot water slurry is mixed with sewage sludge (Klarschlamm), whereafter the excess water more easily can be removed from such mixture. Thereafter the remaining solid sludge can be deposited, but the heavy metals and other contaminations are not taken care of and the disposal problems are not solved.